Moved the compression enum items for point cache into a true enum
typedef instead of using #defines.
Cleanup and restructuring: New 'state' struct in PointCache of type
PointCacheState, which holds all the non-user-defined state variables
for baking and validity. This has its own flags, so the state flags are
conceptually separated from the user-defined options.

The old flags are still in the DNA for backward compatibility mapping,
with an underscore prefix and _DEPRECATED suffix so they are not used
accidentally and name collision is avoided.